I "inspect" my brakes and bearings about every 7 years. Why 7 years? Experience.
I find out that the magnets are done about every 10 years so I "inspect" them and see what they look like at 7 years. I have never repacked the bearing unless I do a brake job. A repack is just not needed "for me." In over a dozen trailers I have owned (not all TT'er) I have never lost a bearing.
"For me" the shoe's last forever. I think on my 30 YO trailer I replaced the shoe's once. If I remember right I think they were contaminated and not wore out.
This is what "I" do and YMMV because driving styles make a big difference.